<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        body,
        html {
            height: 100%;
        }
        
        .box {
            height: 100%;
            display: flex;
            flex-direction: column;
        }
        
        .foot,
        .header {
            height: 50px;
            border: 1px solid red;
        }
        
        .body {
            flex: 1 1;
            border: 1px solid blue;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="header">header</div>
        <div class="body">body</div>
        <div class="foot">foot</div>
    </div>
</body>

</html>